1. Field of the Invention
The present invention relates to a thin plate-reinforcement structure utilizing a stiffening or reinforcing effect of a weld bead, and a method of producing the thin plate-reinforcement structure.
2. Description of the Related Art
Heretofore, as illustrated in FIGS. 10A and 10B, there has been known a welded structure, for example, for a seat back of an automobile seat, in which right and left end faces 31a of a lower frame 31 are bridgingly arc-welded, respectively, to right and left planar portions 30a of a pipe frame 30, wherein a continuous weld bead 32 to be provided along an upwardly and downwardly extending edge of each of the end faces 31a of the lower frame 31 is formed to additionally extend onto a respective one of the planar portions 30a of the pipe frame 30 (see JP 62-259676A: hereinafter referred to as Patent Document 1).
The reason for providing an extension portion 32a to the weld bead 32 is to give a reinforcing function equivalent to a boss or a rib to a welded connection area, thereby preventing deformation in the planar portion 30a and the lower frame 31, and fatigue crack which would otherwise be caused by the deformation.
The planar portions 30a of the pipe frame 30 in the Patent Document 1 are formed by flattening respective opposite lower ends of the pipe frame 30. The pipe frame 30 has been commonly prepared using a pipe having a wall thickness of about 1.5 mm. Thus, it can be said that the planar portion 30a is composed of two superimposed plates each having a thickness of about 1.5 mm. The lower frame 31 has also been commonly prepared using a plate having a thickness of about 1.5 mm.
As a seat back frame, such a plate having a thickness of about 1.5 mm belongs to the category of “thick plate”.
Meanwhile, in late years, as applied to FIG. 10A, a back seat frame 30 has been employed which comprises a pipe frame provided in the upper range A, and a side frame provided in the lower range B, wherein the side frame is prepared as a press-formed product and welded to a lower end of the pipe frame.
With a view to facilitating a reduction in weight of the side frame, a press-formed product is employed which is prepared by superimposing two plates each having a thickness of about 0.5 mm, and reinforcing the superimposed plates by a concavoconvex rib configuration. Similarly, with a view to facilitating a reduction in weight of the lower frame 31, a press-formed product is employed which is prepared by reinforcing a plate having a thickness of about 0.5 mm, by a concavoconvex rib configuration.
As a seat back frame, such a plate having a thickness of about 0.5 mm belongs to the category of “thin plate”. Further, as in an aftermentioned embodiment, there is a seat cushion in which a side frame of a cushion frame is formed using a “thick plate”, and a cushion pan is formed using a “thin plate”.
Thus, in the case where an extension portion 32a of a weld bead 32 is provided during welding between two thick plates as in the Patent Document 1, a direction and length of the extension portion of the weld bead 32 do not matter as much, as long as the extension portion can give a reinforcing function equivalent to a boss or a rib to a welded connection area. For this reason, the Patent Document 1 simply mentions that: the weld bead 32 is formed to additionally extend along an upwardly and downwardly extending edge of the end face 31a of the lower frame 31, as to the direction of the extension portion 32a of the weld bead 32; and the weld bead may additionally extend only by a small length (about 15 mm), as to the length of the extension portion 32a. 